Method of preventing rust on steel surface

ABSTRACT

METALS ARE PROTECTED AGAINST RUST FORMATION THEREON BY IMMERSING THEM IN A MOLYBDATE SOLUTION.

United States Patent Int. Cl. 0532 7/00 US. Cl. 148-6.14 3 Claims ABSTRACT OF THE DISCLOSURE Metals are protected against rust formation thereon by immersing them in a molybdate solution.

RELATED APPLICATION This application is a continuation-in-part of application Ser. No. 683,028, filed Nov. 14, 1967, now abandoned.

FIELD OF INVENTION The present invention relates to a process for treating surfaces of certain metals to improve the corrosion resistance thereof by forming a rust-proof film on the metal surface by immersing the metals, especially steel and steel pipes, in certain molybdate solutions.

BACKGROUND OF INVENTION Heretofore, rust-proofing processes have been practiced by immersing the metals in a chromate solution to prevent red rust from forming on cold-rolled steel plates during storage. While most prior rust prevention has been effected by chomate treating, metals so treated do not have satisfactory adherence with paints when painted thereafter.

SUMMARY OF INVENTION In accordance with the present invention, there is provided a process for rust-proofing steel comprising immersing the steel in an alkaline earth metal molybdate solution. The rust-proof film according to this treatment has comparatively good adhesive property with paints. The treatment is very easily conducted only by immersing a steel in a molybdate solution having a definite concentration at a definite temperature for an arbitrary time after the steel is subjected to degreasing and acid pickling if necessary. The concentration of magnesium molybdate solution and calcium molybdate solution is preferably within the range from 0.025 molar to 0.1 molar and proper temperature is within the range from the normal (room) temperature to 80 C.

It is proved in observation with an electron microscope that the film obtained in accordance with the present invention comprises y-Fe o Fe O Mo O (0H) M00 The present invention may be applied to plain carbon steels, low alloy steels and high alloy steels, etc. Alkali molybdate solution is not proper for a molybdate solution because the corrosion resistance of a steel treated with alkali molybdate solution is not excellent.

The invention is illustrated and in no sense limited by the following examples.

EXAMPLE 1 Cold-drawn steel plate is treated with a molybdate soludate solution by immersing it into a solution consisting of $5 molar of magnesium molybdate, calcium molybdate or sodium molybdate at 60 C. for such a time; as shown in 3,703,418 Patented Nov. 21, 1972 ice Table l which shows the results of immersion test and wetting test for such reagents.

The tests are conducted under the following conditions;

In the immersion test in running water, the decrease in weight (mg/cm?) resulted from the immersion of samples in a water tank having dimensions of 1.0 m. x 4.0 m. x 0.5 m. (m.=meter) with a flow rate of 125 cubic centimeter per minute for 10 days.

Wetting test was carried out by leaving samples of the plate in a tank maintained at a temperature of 50 C. and a humidity of 98% for 24 hours.

Degree of corrosion is shown by the following ratings:

A-no red rust was produced, B-about 2-3 red rust was produced, C-red rust was produced all over the surface.

TABLE 1.CORROSION RESISTANCE WITH MOLYBDA'IE SOLUTION TREATMENT Immersion Treating test, decrease Wetperlod, in weight ting Reagent min (mg. lcm!) test Magnesium molybdate g 2. 26 B A 8 2. 35 1 10 2. 51 A 20 2.10 30 2. 28 A l 30 2. 56 A Calcium molybdate 2 2. 40 B 5 2.14 B 8 2.02 10 2. 01 B 20 2. 54 30 2. 25 A 3 30 2. 06 A Sodium molybdate 2 2. C 5 2. 66 C 8 2. 88 10 2. C 20 2. 70 B 30 B 2 30 2. 86

Non-treated l 3 3. 04 C 1 Plate used in Example 2. 2 Hours. 8 Average for 3 sheets.

As shown in Table l, the effect of treatment varies according to the molybdate solution, and different values are shown. In the immersion test, it is proved that corrision resistance is most excellent in a steel treated with alkali earth molybdate solution, next excellent in a steel treated with sodium molybdate solution, and inferior in a nontreated steel. Further, the wetting test shows the same results. Thus, it is proved that alkali earth molybdate solution is proper for a molybdate solution but alkali molybdate solution is not proper.

EXAMPLE 2 Adherence with paint test is conducted on a non-treated steel plate, chromate treated steel plate which is commonly marketed, and the present invention steel plate. Generally, non-treated steel plate has bad corrosion resistance but is excellent in its adherence with paints. Therefore, such a steel is used as a sample in the test in order to show excellent adherence with paints. Steels subjected to surface treatment generally shows excellent corrosion resistance and bad adherence with paints. Therefore, chromate treated steel plate is used in the test in order to show bad adherence with paints. As steels treated with alkali molybdate solution show bad corrosion resistance as shown in Example 1, which is not proper for a condition of surface treatment, adherence with paint test is not conducted on such a steel. When a non-treated :1 ste'elplate'and a steel plate treated alkali earth molybdate solution show substantially the same adherence, the latter is regarded as having good adherence.

Sample steel plates are painted with baking type acr'ylate resin paint and baking type melamine resin paint with an ordinary method and baked. Then, steel plates thus obtained are tested by cross cut test, drawing test and impact test. In cross cut test, a hundred square incisions having an area of 1 mm. which is deep enough to reach the base plate, are made on the painted surface of the plates to be tested with a diamond needle. Adherence with paints is evaluated by the number of squares left not peeled oif.

In drawing test, a diamond needle in contact with the painted surface of the plates to be tested is loaded at the end thereof with a weight of 700 g. placed on a scale, and the needle is rotated in such a way as to describe a circle having a definite diameter on the surface of the plates to be tested which is moved. In such a way, a continuous arc is described on the surface of the plates. Adherence with paints is evaluated based upon the observation of scratches made on the surface of the plates.

Impact test is conducted with an impact tester with 12.5 mm. in diameter of an impact area, load of 300 g. and a height of 500 mm. Elasticity and adherence of paints are evaluated based upon the results of the impact test.

The rating basis of these tests is shown in Table 2. TABLE 2.RATING BASIS FOR PAINT ADHERING TEST Cross,

cut Rating test Drawing test Impact test 100 No abnormal sign"..- No abnormal Sign. 90-99 Slight peel-off Swollen. 80-89 Peeioii in a small Partial crack.

quantity. 4 50-79 Peel-off in a medium Total crack.

quantity. 1 50 Total peel-oft Total peeloii.

The results of the tests are shown in Table 3.

ample 1 and excellent adherence with paints as shown in Example 2.

As described above, the present method comprises immersing metals, especially steel plates and steel pipes into alkali earth metal molybdate solution, whereupon a rust-proof film such as a molybdenum compound is produced on the surface of the metals. Thus, the corrosion resistance is increased, and rust-proofing is improved. A greater immersion of cold-rolled steel plate into the molybdate solution shows clearly a rust-proof effect. There is no need for high temperature after treatment of the treated steel as with a lithium polymolybdate. The plates or pipes thus treated have excellent adherence with paints, so that this treatment can also be applied as an undercoating for painted metal surfaces.

As indicated by data given above, there is no need for pre-treatment of the steels for corrosion resistance, such as with a solution of a heavy metal salt or with a chromate solution followed by a molybdate solution.

Therefore, the present invention is applicable on an industrial scale.

What we claim is:

1. The process for the provision of a corrosion-resistant film on the surface of steel which consists of immersing the steel, without pre-treatment for corrosion resistance, at a temperature of from room temperature to 80 C. in a solution consisting of an aqueous alkaline earth molybdate solution, wherein the metal is calcium or magnesium, and wherein the concentration of the molybdate solution is from about 0.025 to about 0.1 molar.

2. The process of claim 1 wherein the molybdate is magnesium molybdate.

3. The process of claim 1 wherein the molybdate is calcium molybdate.

TABLE 3.RESULTS OF ADHE RENCE WITH PAINTS TEST [Rating] Kind oi steel plate Treated in Non- 'Ireatedin present P n Test treated chromate invention Cross-cut test 8 1 8 Acrylate resin paint .{Drawing test. 8 1 8 Impact test. 6 6 5 Cross-cut tes 4 1 6 Melamine resin paint ..{Drawing te t; 6 1 1 Impact test 8 4 8 Total results 40 14 43 As shown in Table 3, steel plate treated with chromate References Cited shows poor results, while non-treated steel plate shows UNITED A S A ENTS good results and the present invention steel plate show 2,557,509 6/1951 Miner 1 5 24 the most excellent results. Thus, it is proved that the 3 272,663 9/1966 Kronstein present invention steel plate is excellent in its adherence 248,249 4/1966 Collins X with paints 1,723,067 8/1929 Pacz 1486.14

As may be well understood from the foregoing, steel plate treated with an alkali earth metal molybdate solution has excellent corrosion resistance as shown in Ex- 

